Multi-spectral and hyper-spectral imaging systems, where more than 3 colors are captured by an imager, are of interest for a variety of remote sensing, health, and industrial applications. Multi-spectral imaging (MSI) systems are typically designed with thin-film spectral filters situated in front of detector arrays. MSI systems provide excellent image quality with short detector integration times and modest data sets. They are used in both line-scanned configurations, with linear detector arrays, and full-field configurations, with area detector arrays. The MSI approach can only image a relatively small number of spectral bands, typically 4 to at most 8, and these spectral bands are fixed once the system is built. Hyper-spectral imaging (HSI) systems, on the other hand, use a grating or prism to disperse the various image wavelengths onto an area detector array, providing one spatial axis and one spectral axis. HSI systems are therefore flexible and capable of capturing a vast amount of spectral information, in many very narrow spectral bands. However, compared to MSI systems, the integration times are much longer, the hyper-spectral data sets are extremely large and the spatial resolution is lower. Furthermore, in practice, only a small subset of the captured hyper-spectral data cube is of interest.
Programmable spectrometers and programmable spectral imaging systems, where the spectral transmission function can be modified, have been proposed and implemented using electrically-controlled light modulators in two fundamentally different system configurations: 1) tunable transmissive filters based on liquid crystal devices, acousto-optic devices, or tunable Fabry-Perot cavities and 2) programmable dispersion-based systems with a spatial light modulator consisting of an array of individually addressable devices.
Liquid crystal tunable filters have been disclosed by Miller in U.S. Pat. No. 5,689,317, issued on Nov. 18, 1997, entitled “TUNABLE COLOR FILTER” and by Miller et al. in U.S. Pat. No. 5,892,612, issued Apr. 6, 1999, entitled “TUNABLE OPTICAL FILTER WITH WHITE STATE.” However, liquid crystals are relatively slow (typically, 10-100 msec), have low efficiency and are not space compatible. A programmable spectral imaging system with an acousto-optic tunable filter has been disclosed by Bellus et al. in U.S. Pat. No. 5,828,451, issued Oct. 27, 1998, entitled “SPECTRAL IMAGING SYSTEM AND METHOD EMPLOYING AN ACOUSTO-OPTIC TUNABLE FILTER FOR WAVELENGTH SELECTION WITH INCREASED FIELD OF VIEW BRIGHTNESS.” In general, the tunable filter approach can only pass a single spectral band at a time to the detector array and the spectral bandwidth is predetermined and not tunable.
Programmable dispersion-based systems have been demonstrated using a variety of spatial light modulators, including liquid crystal display panels and, more recently, micro-electromechanical mirror arrays such as the Texas Instruments DMD. In the dispersion-based approach, the input light is sent through a prism or grating to separate the various wavelengths onto the spatial light modulator. The modulator then selects the wavelengths of interest, which are sent to the detector. Most of these dispersion-based systems have been non-imaging or point imaging, utilizing only a single detector element. In a single detector configuration, a 2D image can be generated by raster scanning an object of interest by using, for example, a pair of scanning mirrors.
A non-imaging DMD-based spectrometer for sample analysis is described by R. A. DeVerse et al., “Realization of the Hadamard Multiplex Advantage Using a Programmable Optical Mask in a Dispersive Flat-Field Near-Infrared Spectrometer,” Appl. Spectrosc. 54, pp. 1751-1758 (2000). They show that, when many narrow spectral bands are of interest, signal-to-noise performance can be improved by simultaneously measuring multiple bands using the Hadamard transform approach, rather than measuring the spectral bands sequentially.
An area-imaging programmable spectral imager that does not contain any moving components has been described by C. M. Wehlburg et al., “Optimization and Characterization of an Imaging Hadamard Spectrometer,” Proc. SPIE 4381, pp. 506-515 (2001). Their Hadamard transform spectral imager (HTSI) is a double Offner system with an area detector array. It uses one Offner relay with a curved grating to disperse and reimage the input light onto a DMD. A second Offner relay then de-disperses and reimages the selected components onto the detector array. There are a number of issues with this configuration. 1) The DMD design is not well suited for an Offner system because the DMD is designed for off-axis illumination. With proper illumination and on-axis collection optics, the DMD produces high contrast modulation. However, in the double Offner configuration, the illumination and collection conditions are not satisfied and the DMD contrast would be low. The imaging performance of the double Offner with a DMD is also questionable because of the tilted image planes associated with the tilt angles of the micro-electromechanical mirrors. 2) The curved grating in the Offner is difficult to fabricate and very difficult to blaze for high diffraction efficiency. Therefore, Offner spectrometers have low efficiency and a double Offner spectrometer would have very low efficiency. 3) The HTSI system is designed for imaging of relatively small objects, i.e., objects that are small compared to the size of the detector array. If the HTSI system were used for extended objects, the spectral cross-talk between various parts of the object would be severe. For example, if the object fills (or overfills) the HTSI detector array, a single wavelength image of the object would fill approximately ½ of the DMD. Under this condition, spectral cross-talk in the HTSI would prevent capture of a simple 3-color image.
Recently, an electromechanical conformal grating device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed by Kowarz in U.S. Pat. No. 6,307,663, issued on Oct. 23, 2001, entitled “SPATIAL LIGHT MODULATOR WITH CONFORMAL GRATING DEVICE.” Display systems based on a linear array of GEMS devices were described by Kowarz et al. in U.S. Pat. No. 6,411,425, entitled “ELECTROMECHANICAL GRATING DISPLAY SYSTEM WITH SPATIALLY SEPARATED LIGHT BEAMS,” issued Jun. 25, 2002 and by Kowarz et al. in U.S. Pat. No. 6,678,085, entitled “HIGH-CONTRAST DISPLAY SYSTEM WITH SCANNED CONFORMAL GRATING DEVICE,” issued Jan. 13, 2004. The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of '663 has more recently become known as the conformal GEMS device or, more simply, GEMS device, with GEMS standing for grating electromechanical system. The GEMS device possesses a number of attractive features. It has a large active region that provides high-speed digital light modulation with high contrast and good efficiency for on-axis illumination. In addition, the device can be fabricated at low cost with relatively few masks in a CMOS or CCD foundry.
There is a need therefore for a high-resolution imaging system that has a programmable spectral transmission function and can rapidly image large extended objects. There is also a need for a programmable spectral imaging system that provides good spectral selectivity with high efficiency over a large spectral range. There is a further need for a programmable spectral imaging system that can employ electromechanical grating devices.